1. Field of the Invention
The present invention relates to an information recording/reproducing apparatus using an optical recording medium such as an optical disk.
2. Description of the Related Art
Recording and reproducing systems for an optical disk can be considered as a kind of low-pass filters in view of the OTF (Optical Transfer Function) of an objective lens and the response characteristic or the like of a recording material. If information is recorded with a high linear recording density increased to near the upper limit of the frequency response of the recording and reproducing systems, a read error occurs due to a so-called code interference by which reproduced waveforms are likely to interfere with each other at the time adjacent marks are read.
Suppose a train of signals "0,1,1,0,1" is input while considering the recording and reproducing systems for an optical disk as a single transfer path. If an input "1" is a pulse with a level of "1" , the train of signals "0,1,1,0,1" is the input signal as shown in FIG. 1. The output shown in FIG. 1 is acquired in accordance with this input signal. The signal indicated by the solid line is output first according to the first input pulse, the signal indicated by the broken line is then output according to the next input pulse, and the signal indicated by the alternate long and short dash line is output according to the last input pulse. The final output therefore becomes the sum of the former two outputs as indicated by the alternate long and two short dashes line. The acquired train of output signals, "0,1,2,1,1" , differs from the train of input signals.
Because of the above characteristic, with a high linear recording density accomplished, the recording and reproducing systems for an optical disk can be said to be approximated to a PR (1,1) transfer path (Partial Response Transfer Path) as shown in FIG. 2. The PR (1,1) transfer path shows a characteristic such that, when a train of input signals has a digitized value, the output has a ternary value.
Conventionally, therefore, by using a method of ternary detection while considering the recording and reproducing systems for an optical disk is considered as a PR (1,1) transfer path, information recording and reproduction on and from an optical disk designed to have a high linear recording density are accomplished by apparatuses having structures as shown in FIGS. 3 and 4.
FIG. 3 illustrates the structure of a conventional information recording apparatus for an optical disk.
In recording information, record information digitized to have values "0" and "1" is supplied to a precoder 1. The precoder 1 modulates data before inputting the data to the PR (1,1) transfer path, and is equivalent to a transfer path which processes input data with the opposite characteristic to that of the PR (1,1) transfer path, as shown in FIG. 5. The output of the precoder 1 is supplied via a digitizer 2 to an optical head 3. The digitizer 2 digitizes other values than "0" and "1" which are produced by the precoder 1, and may be designed to divide an input value by "2" and output the remainder. The optical head 3 performs photoelectric conversion on the output of the digitizer 2 and records the resultant signal on an optical disk which is rotated by a spindle motor 4. It is known that the precoder 1 and digitizer 2 have a so-called NRZi modulation (Non Return to Zero Inverting) function that inverts the level of an output signal when an input signal is "1" . The record information is therefore considered to be supplied to the optical head 3 via an NRZi modulator 6, which comprises the precoder 1 and the digitizer 2.
FIG. 4 illustrates the structure of a conventional information reproducing apparatus for an optical disk.
In reproducing information, an optical pickup 7 accesses the optical disk 5 for information reading, yielding a reproduced signal. The reproduced signal is amplified by an equalizer amplifier 8 before it is supplied to a level detector 9. The level detector 9 digitizes the ternary reproduced signal to restore it to the original record information and outputs the reproduced information. The process the optical pickup 7 performs to read information from the optical disk 5 is equivalent to the PR (1,1) transfer path due to the code interference. The reproduced signal therefore is a ternary signal having values of "0", "1"and "2". The level detector 9 compares the level of the signal from the equalizer amplifier 8 with first and second threshold levels. The level detector 9 is designed to output "0" when the level of the signal from the equalizer amplifier 8 is lower than the first threshold level or equal to or higher than the second threshold level, and output "1" when the level of that signal is equal to or higher than the first threshold level and lower than the second threshold level.
In short, the conventional recording and reproducing systems for an optical disk designed to have a high linear recording density should separate and extract a ternary signal using two threshold levels. This separation/extraction of a signal accurately converted to have three values requires a higher S/N ratio in the reproduced signal than the separation and extraction of a digital signal by a single threshold level, and is therefore practically difficult to accomplish.